Conventionally, a Cu—Fe—P alloy is generally used as a copper alloy for semiconductor lead frames. As these Cu—Fe—P alloys, for example, a copper alloy (C19210 alloy) containing Fe 0.05 to 0.15% and P 0.025 to 0.040%; and a copper alloy (CDA194 alloy) containing Fe 2.1 to 2.6%, P 0.015 to 0.15%, and Zn 0.05 to 0.20%, can be exemplified. When intermetallic compounds such as Fe and Fe—P are precipitated in the copper matrix phase, these Cu—Fe—P alloys are excellent in its strength, electric conductivity, and thermal conductivity among copper alloys; hence they are widely used as international standard alloys.
The recent advancement of the large-capacity, miniaturization, and high-performance of semiconductor devices used in electronic apparatuses has urged the growing reduction in the cross-sectional area of lead frames adopted in the semiconductor devices; thereby there is a demand for a higher strength, electric conductivity, and thermal conductivity. With the demand, there is also a demand for a higher strength and a higher thermal conductivity against a copper alloy sheet used in the semiconductor devices.
On the other hand, the copper alloy sheets provided with a high strength are also requested to have the workability to be formed into the lead frame with the reduced cross-sectional area. Specifically, a copper alloy sheet is subjected to a stamping process so as to be formed into a lead frame, hence, a copper alloy sheet is requested to have an excellent stampability. The request is also made when a copper alloy sheet is used for applications of being press-punched other than the application of the lead frames.
Conventionally, in order to improve a stampability of a Cu—Fe—P alloy sheet, the following measures have been widely used. The measures are as follows: control of chemical components in which trace additives, such as Pb and Ca, or a compound that could be a starting point of a break, are to be dispersed; or control of a grain size or the like.
However, these measures have problems in that the controls per se are difficult to be carried out, these controls adversely affect other properties, and a production cost is therefore increased.
Contrary to that, focusing attention on a microstructure of a Cu—Fe—P alloy sheet, it is proposed that a stampability and a bending workability thereof are improved. For example, Patent Document 1 discloses a Cu—Fe—P alloy sheet containing Fe 0.005 to 0.5 wt % and P 0.005 to 0.2 wt %, and further contains either one or both of Zn 0.01 to 10 wt % and Sn 0.01 to 5 wt %, if needed. According to Patent Document 1, a stampability is improved by controlling an integration degree of a crystal orientation of the copper alloy sheet (see Patent Document 1).
More specifically, in Patent Document 1, the integration degree is controlled by the use of the fact that: as the copper alloy sheet is recrystallized and its grain size becomes larger, the orientation density in the {200} plane and the {311} plane in the sheet surface are larger; and when the copper alloy sheet is rolled, the orientation density in the {220} plane is larger. Characteristically, Patent Document 1 is intended to improve the stampability by increasing the orientation density in the {220} plane in the sheet surface relative to the {220} plane and the {311} plane. More specifically, assuming that an intensity of X-ray diffraction of {200} plane in the sheet surface is I[200], that of {311} is plane I[311], and that of {220} plane is I[220] the document specifies that [I[200]+I[311]]/I[220]<0.4 should be satisfied.
Patent Document 2 proposes that, in order to improve a stampability, a ratio (I(200)/I(220)) of the intensity I(200) of X-ray diffraction of (200) plane in a copper alloy sheet to the intensity I(220) of X-ray diffraction of the (220) plane, should be 0.5 or more to 10 or less; or the orientation density of Cube orientation (D(Cube orientation)) should be 1 or more to 50 or less; or a ratio (D(Cube orientation)/D(S orientation)) of the orientation density of D orientation (D(Cube orientation)) to the orientation density of S orientation (D(S orientation)) should be 0.1 or more to 5 or less (see Patent Document 2).
Further, Paten Document 3 proposes that, in order to improve a bending workability of a Cu—Fe—P alloy sheet, a ratio ([I(200)+I(311)]/I(220)) of a total of the intensity of X-ray diffraction of the (200) plane and that of the (311) plane to the intensity of X-ray diffraction of the (220) plane, should be 0.4 or more (see Patent Document 3).
Further, Patent Document 4 proposes that, in order to improve a bending property of a Cu—Fe—P alloy sheet, I(200)/I(110) should be 1.5 or less (see Patent Document 4).
On the other hand, a Cu—Fe—P alloy sheet with a high strength is required to have a high softening resistance such that it is hardly decreased in the strength even when subjected to a heat treatment, such as a stress relief annealing.
Generally, a lead frame having a plurality of pins is fabricated by subjecting a Cu—Fe—P alloy sheet to a stamping process (press punching process). As stated above, in a copper alloy sheet used as a material for electric and electronic parts, thinning of the copper alloy sheet and increase in the numbers of pins have progressively advanced in recent years to cope with the miniaturization, the thinning and weight reduction of the parts. With the advancement, residual stresses are liable to remain in such a lead frame after subjected to the stamping process and the pins thereof tends to be arranged irregularly. Therefore, a copper alloy sheet with a plurality of pins formed by the stamping process is usually subjected to a heat treatment (stress relief annealing) such that stresses are relieved.
However, when subjected to such a heat treatment, a material tends to be softened, and cannot maintain the mechanical strength before the treatment. In addition, from a viewpoint of improving productivity in production processes, the treatment is required to be performed at a higher temperature and in a shorter time, hence there is a strong demand for a softening resistance with which the material can maintain a high strength after subjected to a heat treatment at a higher temperature.
To cope with these problems, some measures have been taken so far in which alloy elements, such as Fe, P, and Zn, and other additive trace elements, such as Sn, Mg, and Ca, are to be contained, or additive amounts of these elements are adjusted. Controls of dispersoids and precipitates in a copper alloy sheet have also taken so far. However, only with such adjustments of elements or controls of dispersoids and precipitates, a copper alloy sheet cannot fully cope with the growing miniaturization and thinning of copper alloy parts or the desired softening resistance property, hence other techniques are further proposed in which microstructures or the like of a copper alloy sheet is controlled.
For example, a technique disclosed in Patent Document 5 increases a strength of a copper alloy, which is not a Cu—Fe—P alloy but used as a material and produced by adding a small amount of Ag in an oxygen free copper, by controlling a X-ray diffraction intensity ratio after the final rolling and controlling a grain size before the final rolling. That is, a copper alloy with a high strength is obtained by the following measures: after subjected to a hot-rolling, the copper alloy is subjected to a plurality of working cycles each of a cold-rolling and a recrystallization annealing; and a reduction ratio in the final rolling, an average grain size after subjected to the recrystallization annealing before the final cold-rolling, and a reduction ratio in the cold-rolling before the final annealing, are controlled such that the X-ray diffraction intensity ratio after the final rolling, and the grain size before the final rolling are controlled. However, even when applying the rolling and annealing conditions that the document recommends, to the Cu—Fe—P alloy targeted by the present invention, as they are, such a higher level of softening resistance as requested of the above stated lead frame or the like cannot be acquired (see Patent Document 5).
Contrary to that, various techniques for improving a softening resistance in a Cu—Fe—P alloy have been proposed. For example, a technique of Patent Document 6 proposes that a high softening resistance can be acquired by controlling forms per se of a dispersoid and a precipitate of a Cu—Fe—P alloy with a substantial Fe content of 0.7% or more, which is a large content. That is, a higher softening resistance can be acquired by the measures that a ratio(Xγ/Xα) of the X-ray peak area (Xγ) of the γ-Fe crystallized substance contained in a microstructure, to the X-ray peak area (Xα) of the α-Fe crystallized substance contained therein, is 0.05 or more (see Patent Document 6).
A technique of Patent Document 7 proposes that, in order to acquire a higher softening resistance by controlling a texture, the orientation density of Cube orientation in a Cu—Fe—P alloy with a substantial Fe content of 0.5% or more, which is a large content, after subjecting the copper alloy to an annealing at 500° C. for 1 minute, should be 50% or less; and further an average grain size thereof should be 30 μm or less (see Patent Document 7).
A technique of Patent Document 2 discloses that a Cu—Fe—P alloy with a substantial Fe content of 2% or more, which isa large content, can be improved in its workability of the sheet and formability into a lead frame by controlling its texture, but not intended to improve its softening resistance. Herein, the workability means a corrugation, meandering and uneven residual stress of the sheet, in a cold-rolling; a slit streak; occurrence of a skew and burr in a stamping process; and a rough surface and crack in a lead bending processed portion. In addition, the texture means that the X-ray diffraction intensity ratio of the (200) plane and the (220) plane and the orientation density of Cube orientation, are properly controlled.
On the other hand, in the plastic packages for semiconductor devices, the package in which a semiconductor chip is encapsulated by a thermosetting resin is a mainstream, because the package is excellent in the economic efficiency and mass productivity. With the recent demands for miniaturization of electronic parts, the package becomes increasingly thinner.
When assembling the package, semiconductor chip is heated to be adhered to a lead frame by using an Ag paste, etc., or soldered or brazed with Ag via a plated layer made of AU or Ag or the like. After that, the package is generally encapsulated with a resin, subsequently an implementation is performed on an outer lead by an electroplating.
The most serious problem concerning the reliability of these packages is a package crack or peeling occurring upon the implementation. Peeling of a package occurs by a thermal stress generated in the subsequent heat treatment, when a resistance of peel off between a resin and a die pad (portion where a semiconductor chip of a lead fame is mounted) is deteriorated after assembling the semiconductor package.
Contrary to that, a package crack occurs through the following processes: after assembling a semiconductor package, a mold resin absorbs moisture from the air, and the moisture vaporizes by heating in the subsequent surface implementation. When a crack is present inside the package at the time, the moisture is applied to the peeled plane, which acts as an internal pressure. A swelling is caused in the package by the inner pressure, or a crack is caused when the resin is weak against the inner pressure. When a crack is caused in a package after the surface implementation, moistures and impurities are incursive therein to cause the chip to be corroded; hence impairing a function as a semiconductor. In addition, the swelling of a package results in a poor appearance and lost of its commodity value. Such problems involving package cracks and peelings have recently been remarkable with the advancement of thinning of the packages stated above.
The problems involving package cracks and peelings are caused by the deteriorated adhesion property between resins and die pads. An oxide film of a lead frame base material has the greatest influence on the resistance of peel off between the resin and the die pad. The lead frame base material has been subjected to various heating processes for producing the sheet or the lead frames. Accordingly, an oxide film with a thickness of several tens to several hundreds of nanometers is formed on the surface of the base material before the plating process by Ag or the like. On the surface of the die pad, a copper alloy and the resin are in contact with each other via the oxide film, hence the peeling of the oxide film from the lead frame base material directly leads to the peeling between the resin and the die pad, causing the resistance of peel off between the resin and the lead frame base material to be remarkably decreased.
Accordingly, the problem involving the package crack and the peeling depends on the resistance of peel off between the oxide film and the lead frame base material. Therefore, the above stated Cu—Fe—P alloy with a high strength is required as a lead frame base material to have a high resistance of peel off of the oxidation film formed on its surface through various heating processes.
With respect to such problem, many measures have not been proposed so far; however, Patent Document 8 proposes that the resistance of peel off of the oxidation film can be improved by controlling a crystalline orientation in the surface layer of a copper alloy pole. That is, Patent Document 8 proposes that, in a crystalline orientation in a pole surface evaluated by the thin film method using an XRD of the lead frame base material copper alloy, the resistance of peel off of the oxidation film can be improved by the measures that a ratio of the peak intensity of {100} to the peak intensity of {111} should be 0.04 or less. It is noted that Patent Document 8 includes every kind of copper alloys for lead frames; however, the Cu—Fe—P alloys substantially exemplified are only a Cu—Fe—P alloy with Fe content of 2.4% or more, which is a large content.
[Patent Document 1] Japanese Patent Laid-Open No. 2000-328158 (entire description)
[Patent Document 2] Japanese Patent Laid-Open No. 2002-339028 (entire description)
[Patent Document 3] Japanese Patent Laid-Open No. 2000-328157 (Claims for the Patent)
[Patent Document 4] Japanese Patent Laid-Open No. 2006-63431 (Claims for the Patent)
[Patent Document 5] Japanese Patent Laid-Open No. 2003-96526 (entire description)
[Patent Document 6] Japanese Patent Laid-Open No. 2004-91895 (entire description)
[Patent Document 7] Japanese Patent Laid-Open No. 2005-139501 (entire description)
[Patent Document 8] Japanese Patent Laid-Open No. 2001-244400 (entire description)